If Bowles’s analysis is right, it suggests that individuals who could not conform, or who were disruptive, would have weakened the whole group; any group that failed to drive out such people, or kill them, would have been more likely to be overwhelmed in battle. Conversely, people who fit in—sharing the food they found, joining in hunting, helping to defend the group, and so on—would have given their group a collective advantage, and thus themselves an individual evolutionary advantage.

This suggests two hypotheses. First, that one of the traits that may have evolved in humans is conformity, an ability to fit in with a group and adopt its norms and customs. Second, that enforcement of those norms and customs could have been essential for group cohesion and harmony, especially as groups got bigger (bigness is important in battles against other groups).

Let’s start with conformity. This hasn’t been studied much in other animals, but male baboons do appear to conform to the social regimens of the groups they join. For example, in one baboon troop in Kenya in the 1980s, all the aggressive males died of tuberculosis. The aggressives were the ones to snuff it because they’d eaten meat infected with bovine TB that had been thrown into a garbage dump; only the more-aggressive males ate at the dump. After their deaths, the dynamics of the troop shifted to a more laid-back way of life. Ten years later—by which time all the original resident males had either died or moved on—the troop was still notable for its mellow attitude. The new males who’d arrived had adopted the local customs.

What about humans? According to Michael Tomasello—a psychologist at the Max Planck Institute, in Leipzig, Germany, who studies the behavior of human children and of chimpanzees—children as young as 3 will quickly deduce and conform to rules. If an adult demonstrates a game, and then a puppet comes in and plays it differently, the children will clamor to correct the puppet with shouts of “No, that’s not the way you do it—you do it this way!” In other words, it’s not just that they infer and obey rules; they try to enforce them, too.

Which brings me to the question of punishment.

Punishment Games

I’ll be dictator. Here’s how we play: An economist puts some money on the table—let’s say $1,000. Since I’m dictator, I get to decide how you and I are going to split the cash; you have no say in the matter. How much do you think I’ll give you?

Now, let’s play the ultimatum game. We’ve still got $1,000 to play with, and I still get to make you an offer. But the game has a wrinkle: If you don’t like the offer I make, you can refuse it. If you refuse it, we both get nothing. What do you think I’ll do here?

As you’ve probably guessed, people tend to play the two games differently. In the dictator game, the most common offer is nothing, and the average offer is around 20 percent. In the ultimatum game, the most common offer is half the cash, while the average is around 45 percent. Offers of less than 25 percent are routinely refused—so both players go home empty-handed.

Economists scratch their heads at this. In the first place, they are surprised that some people are nice enough to share with someone they don’t know, even in the dictator game, where there’s nothing to lose by not sharing. Second, economists predict that people will accept any offer in the ultimatum game, no matter how low, because getting something is better than getting nothing. But that’s not what happens. Instead, some people forgo getting anything themselves in order to punish someone who made an ungenerous offer. Money, it seems, is not the only currency people are dealing in.

Bring in the neuroscientists, and the other currency gets clearer. If you measure brain activity while such games are being played (and there are many variants, for the fun doesn’t stop with dictators and ultimatums), you find that the reward centers of the brain—the bits that give you warm, fuzzy feelings—light up when people are cooperating. But they also light up if you punish someone who wasn’t generous, or watch the punishment of someone who wasn’t.

Whether these responses are universal isn’t clear: The genetic basis is obscure, and the number of people who’ve had their brain activity measured is tiny. Moreover, most economic-game playing has been done with college students; the extent to which the results hold among people from different cultures and backgrounds is relatively unknown. But the results suggest an intriguing possibility: that humans have evolved both to be good at conforming to the prevailing cultural norms and to enjoy making sure that those norms are enforced. (Perhaps this explains why schemes such as zero-tolerance policing work so well: They play into our desire to conform to the prevailing norms.)

Bringing Out the Best

If the evolutionary scenario I’ve outlined is even half right, then we should expect to find that there are genes involved in mediating friendly behavior. And there are. Consider Williams syndrome.

People who have Williams syndrome tend to have poor cardiovascular function and a small, pointed, “elfin” face. They are typically terrible with numbers but good with words. And they are weirdly, incautiously friendly and nice—and unafraid of strangers.

They are also missing a small segment of chromosome 7. Chromosomes are long strings of DNA. Most people have 46 chromosomes in 23 pairs; you get one set of 23 from your mother, and the other from your father. In Williams syndrome, one copy of chromosome 7 is normal; the other is missing a small piece. The missing piece contains about 20 genes, some of which make proteins that are important in the workings of the brain. Since one chromosome is intact, the problem isn’t a complete absence of the proteins that the genes encode, but an insufficiency. Somehow, this insufficiency results in people who are too nice. What’s more, they can’t learn not to be nice. Which is to say, someone with Williams syndrome can learn the phrase “Don’t talk to strangers” but can’t translate it into action.

Much about Williams syndrome remains mysterious. How the missing genes normally influence behavior is unclear; moreover, the environment has a role to play, too. But despite these complexities, Williams syndrome shows that friendliness has a genetic underpinning—that it is indeed as primal as ferocity. Indirectly, it shows something else as well. Most of us are able to apply brakes to friendly behavior, picking and choosing the people we are friendly to; those with Williams syndrome aren’t. They cannot modulate their behavior. This is even odder than being too friendly. And it throws into sharp relief one of the chief features of ordinary human nature: its flexibility.

One of the most important, and least remarked upon, consequences of social living is that individual behavior must be highly flexible and tailored to circumstance: An individual who does not know whom to be aggressive toward, or whom to help, is unlikely to survive for long within the group. This is true for baboons and chimpanzees. It is also true for us.

Indeed, the ability to adjust our behavior to fit a given social environment is one of our main characteristics, yet it’s so instinctive we don’t even notice it, let alone consider it worthy of remark. But its implications are profound—and hopeful. It suggests that we can, in principle, organize society so as to bring out the best facets of our complex, evolved natures.